There are a number of techniques currently available to determine the geographical location of a radio device within a radio communication system. One technique of positioning a radio device is the time difference of arrival (TDOA) method. TDOA systems operate by placing locating units at geographically disbursed locations within a coverage area of the locating devices. The locating units may be stand-alone units or may be integrated with a base station of a wireless telephone network. For example, the coverage area may include a portion of a wide area wireless network or a portion of a local area wireless network.
The locating units operate in either a signal reception mode to receive a signal from the radio device or in a signal transmission mode to transmit a signal to the radio device. When the locating units are configured to receive a signal transmitted from the radio device, each locating unit timestamps the reception of the signal. The differences in the arrival times between the locating devices are compared using, for example, correlation methods, to produce intersecting hyperbolic lines from which the geographical location of the radio device is estimated. When the locating units are configured to transmit respective signals to the radio device, the radio device timestamps the arrival of each of the respective signals. The time difference of rival between the timestamps, along with the position of each of the locating units, is used to calculate the geographical location of the radio device.
Regardless of the TDOA mode, the locating units must be accurately synchronized to each other. For example, in signal reception mode, the local clocks of the locating units must be synchronized in time to facilitate a meaningful comparison of the timestamps. In signal transmission mode, the local clocks of the locating units must be synchronized to control the timing of transmission of the signal from each of the locating units. Typically, time synchronization is accomplished by periodically adjusting the local clock of each locating unit to synchronize the local clocks with a reference clock.
Three common techniques for time synchronization include providing dedicated cabling between the locating units, connecting the locating units to a computer network using a protocol, such as IEEE 1588 and using an external time reference, such as a Global Positioning System (GPS) clock. However, using dedicated cabling presents the obvious disadvantage of purchasing and installing the dedicated cabling, especially when the locating units are geographically distant from one another. In addition, highly accurate time synchronization may not be possible when using a network with multiple switches and/or routers between the locating units. For example, when using a T1/E1 link with multiple switches/routers, inbound signals may traverse a different path from outbound signals, thereby producing different propagation delays in inbound and outbound traffic.
Furthermore, although GPS enables highly accurate time synchronization, GPS requires each locating unit to include an expensive GPS receiver, which may be cost-prohibitive. In addition, GPS only works in areas where the GPS receiver has access to a sufficient number of GPS satellites. For example, GPS synchronization may not be possible for locating units that are located in remote areas, in the interior of a building or underground.
As a result, there is a need for a time synchronization system that synchronizes the local clocks of locating units without dedicated cabling, networking or expensive GPS receivers.